1. Field of the Invention
The present invention generally relates to an optical transmission system and, more particularly, relates to optical filters for accomplishing a change of the gain characteristics of an optical amplifier over an operating wavelength band.
2. Technical Background
In an optical network, optical signals are typically transmitted through a fiber over relatively long distances. Because the strength of the optical signals tends to decrease with increasing transmission path length, it has become commonplace to divide the fibers into spans, with in-line optical amplifiers positioned between the spans. The typical span is, for example, 80-120 km in length. While the in-line optical amplifiers boost the signal strength of the transmitted optical signals, such optical amplifiers typically do not exhibit flat gain characteristics over the band of wavelengths of the optical signals that are transmitted through the optical amplifier. Thus, in an optical network, if each of the optical amplifiers positioned between each fiber span amplify optical signals having certain wavelengths more than they amplify optical signals having other wavelengths, some optical signals will not be amplified sufficiently over a long transmission path and those signals will be more susceptible to errors.
To provide for uniformity of signal amplification at each span of all optical signals transmitted through the network, various techniques have been proposed to flatten the gain of the optical amplifiers so that all the optical signals are amplified the same amount by each optical amplifier provided along a given transmission path. One technique that has been proposed is to provide a gain-flattening dielectric optical filter that has an insertion loss spectrum (also referred to as the xe2x80x9ctransmission spectrumxe2x80x9d) that is inversely related to the gain spectrum of the optical amplifier. In other words, the gain-flattening filter will attenuate those wavelengths that are more greatly amplified by the optical amplifier such that the output of the gain-flattened amplifier exhibits a substantially flat and equal gain for all the wavelengths in the wavelength band of interest.
One of the most important parameters of an optical amplifier, such as an erbium doped fiber amplifier (EDFA), is the slope of the optical gain spectrum. Adjustments to the slope of the optical gain spectrum is often required to accommodate a variety of system operating conditions and amplifier gain transfer functions, such as flattening the gain of the amplifier. The traditional method for achieving slope adjustment is to introduce a variable optical attenuator (VOA) 3 (FIG. 1A) with a spectrally flat response, which is placed in between two amplifier gain sections 2a and 2b. When changing the spectrally flat loss of the VOA, the power, which is coupled from the EDFA gain sections before the VOA to all gain sections following the VOA, changes, thereby affecting the population inversion in those latter stages. This change of the population inversion introduces a change of the gain tilt A, which is defined here as the slope of the best linear fit function y=A*xcex+B to the spectral shape of the gain. This is illustrated in FIGS. 1B-1D. FIG. 1B shows the input spectrum of the received signals, which is assumed, for this example to be flat. FIG. 1C shows the insertion loss spectrum for the VOA for two different states. Both states shown in FIG. 1C have approximately flat loss characteristics across the relevant spectrum. FIG. 1D shows the output gain spectrum of the amplifier for the two different states of the VOA. As apparent from FIG. 1D, a change in the insertion loss spectrum for this VOA results in a gain tilt change in the output gain spectrum. This gain tilt change approximately corresponds to a variation of gain slope.
The disadvantage of a flat spectral VOA is that a significant average power loss change is required to achieve a significant gain tilt change, which is detrimental in at least two ways. Particularly, average power loss change significantly affects the output signal to noise ratio (OSNR) of the amplifier, and, to maintain a constant output power of an EDFA, the pump power would need to be readjusted. Additionally, the maximum gain tilt A of the EDFA (for constant signal input power) is defined by the minimum loss of the VOA. This also implies that the highest population inversion and therefore the best OSNR occurs always for a maximum gain tilt A.
Recently, new VOA devices have been proposed to circumvent these detrimental effects. The new xe2x80x9cslope VOAxe2x80x9d device 4 (FIG. 2A) is similarly positioned between amplifier stages 2a and 2b. In these new xe2x80x9cslope VOAxe2x80x9d devices, which have an approximately linear target spectral response with wavelength, adjustment of the slope of the VOA insertion loss spectrum (FIG. 2C) results in an adjustment of the gain tilt of the amplifier as shown in FIG. 2D.
The mechanism behind the generation of a linear slope change in a xe2x80x9cslope VOAxe2x80x9d is typically the superposition of two sinusoidal response filters, with a nominal phase difference (center wavelength difference) of 180xc2x0 (half the free spectral range (FSR)) between them. By adjusting the relative phase and amplitude away from nominal, an approximately linear response can be generated. The advantage of this new xe2x80x9cslope VOAxe2x80x9d is that the impact on OSNR and pump power readjustment requirements is minimized. This represents an improvement over the flat spectral VOA, but still carries some significant disadvantages. Specifically, these disadvantages include: the linear response is only guaranteed for some maximum phase change, beyond which non-linearity degrades the gain slope changes into a nonlinear gain change; to achieve a certain level of gain slope change, a roughly equal proportion of excess average insertion loss is introduced; and the number of control parameters required to define the attributes of a xe2x80x9cslope VOAxe2x80x9d is significant, since the amplitude coupling coefficients and phases for each Fourier component in the interferometer typically requires an independent control. Additionally, both VOA devices described above require an additional gain-flattening filter in the amplifier device to achieve a spectrally flat output signal or gain transfer function for at least one VOA setting.
Thus, there exists the need for an improved optical filter that: (1) is responsive to fewer control parameters to change the gain slope of an amplifier with which the filter is used, (2) provides a linear gain change throughout the operating wavelength band; and/or (3) has a lower average power loss.
An aspect of the present invention is to provide an optical device having an optical input and output. The optical device comprises an optical amplifier to amplify optical signals received through the optical input of the device, said optical amplifier exhibiting a gain that varies approximately linearly with respect to wavelength in accordance with a gain slope, and a tunable optical filter coupled to the optical amplifier. The optical filter changes the gain slope of the optical amplifier in response to a change in a single parameter of the optical filter.
According to one embodiment of the present invention, the optical filter changes the gain slope of the optical amplifier in response to a change in a central wavelength of the optical filter.
The optical filter may have a pseudo-parabolic spectral filter function. In one embodiment of the invention, the optical filter changes the gain slope of the optical amplifier in response to a change in a central wavelength of the pseudo-parabolic spectral filter function of the optical filter.
According to another embodiment, the single parameter is varied as a function of temperature such that the optical filter compensates for variations in the gain spectrum of the optical amplifier that occur as a function of operating temperature.
Another aspect of the present invention is to provide an optical device having an optical input and output. The optical device comprising an optical amplifier to amplify optical signals received through the optical input of the device, and a tunable optical filter coupled to the optical amplifier. The optical amplifier exhibits a gain that varies approximately linearly with respect to wavelength in accordance with a gain slope. The optical filter changes the gain slope of the optical amplifier in response to a change in a central wavelength of the optical filter.
It is another aspect of the present invention to provide a method of varying the slope of the gain spectrum of an optical amplifier. The method comprising the steps of (a) coupling a tunable optical filter to the optical amplifier, the optical filter having a spectral filter function that is substantially non-linear throughout the operating waveband of the optical amplifier and defines a central wavelength; and (b) adjusting the central wavelength of the optical filter until a desired gain slope adjustment results.
According to another aspect of the present invention, an optical filter is provided for an optical device that exhibits at least two different gain states. The optical filter comprises a tunable filter component that changes its spectral filter function between at least two different states corresponding to the at least two different gain states of the optical device in response to a change in a single parameter of the optical filter.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.